Caspase-8 was originally identified as the first in a series of proteases that link death receptors to the apoptotic process. It is now clear from several studies that caspase-8 is also required for the initiation of proliferation for several cell types, including T lymphocytes. However, the caspase-8 substrate in this process has remained elusive for years. We now propose that the caspase-8 homologue, c-FLIPL, is both the activator of caspase-8 and its initial substrate following T cell activation. Our preliminary findings support a novel model in which T cell receptor (TCR) ligation initiates heterodimerization of caspase-8 with c-FLIPL, which activates caspase-8 through a known activation loop in the C-terminus of c-FLIPL. c-FLIPL is then rapidly cleaved by caspase-8 at Asp376, yielding p43FLIP, which lacks the activation loop for caspase-8. p43FLIP is then able to recruit the adaptor proteins RIP1 and TRAF2 (that c-FLIPL cannot recruit) that promote activation of NF-kB. We thus hypothesize that expression of p43FLIP by transfection and transgenesis will obviate the need for caspase-8 activity in T cell activation of NF-kB and proliferation, and will also limit continual caspase-8 activation that might lead to premature T cell death. A non-cleavable D376A-FLIPL mutant is expected to do the opposite, namely be unable to recruit RIP1 and TRAF2, and will promote excessive caspase-8 activation and premature T cell death. This model will be test by both in vitro signaling studies in Aim 1 and in vivo in Aim 2 to examine the effects on the generation of effector and memory T cells. Public Health Relevance: The coordination of T lymphocyte activation and cell death is fundamental to the immune responses during infection, autoimmune diseases, and lymphomagenesis. A signal pathway involving caspases was previously felt to be involved only with cell death. Now it is clear that caspase-8 is also critical for T cell activation, but the caspase-8 substrate in this function has remained elusive for several years. We propose c-FLIPL as both the activator of caspase and the critical caspase-8 substrate in T cell activation.